Microdrills nowadays have widely been applied to drill micro-holes in various printed circuit boards (PCB). Referring to FIG. 1A to FIG. 1C, the details of a microdrill 50 are described, where FIG. 1A is a schematic lateral view of a microdrill according to an embodiment, FIG. 1B is a sectional view of the microdrill in FIG. 1A along a section-line 1B-1B, and FIG. 1C is a sectional view of the microdrill in FIG. 1A along a section-line 1C-1C. The microdrill 50 is, for example, a standard type drill (ST type drill or called straight type drill), has a central axis 51, and includes a shank 52 and a drill body 54. The drill body 54 includes a drill point 60, helical flutes 58, and a drill tip 60a. The drill body 54 is magnified in scale relative to the shank 52 for ease of illustration. The drill body 54 is composed of the drill point 60 and the helical flutes 58 in function. The drill point 60 is used to produce a drilling action, and the helical flutes 58 are used to remove chips.
In the drill body 54, there is a conical, core which has not been fluted and is called drill web 56, and the thickness of the drill web 56 (called the web thickness 62 hereinafter) conflict with a depth of the helical flute 58 in design. The microdrill 50 with a larger web thickness 62 can lead to good drill rigidity while the depth of the helical flute 58 is smaller, thus resulting in poor chip-removal ability. On the contrary, the helical flute 58 with a larger depth can lead to good chip-removal ability while the drill rigidity thereof is lower. Therefore, the web thickness 62 is a key parameter influencing quality of the microdrill 50. The measurement of the web thickness of microdrill products for improving manufacturing parameters is an important quality management task that microdrill manufacturers concern.
On the other hand, microdrills may be undercut type drills (UC type drills). Referring to FIG. 1D to FIG. 1F, the details of a UC type microdrill 50′ are described, where FIG. 1D is a schematic lateral view of a microdrill according to an embodiment, FIG. 1E is a sectional view of the microdrill in FIG. 1D along a section-line 1E-1E, and FIG. 1F is a sectional view of the microdrill in FIG. 1D along a section-line 1F-1F. The microdrill 50′ includes a shank 52′ and a drill body 54′. The drill body 54′ includes a drill point 60′, helical flutes 58′, and a drill tip 60a′. For the microdrill 50′, the drill body 54′ is ground to yield a UC diameter, i.e. the external diameter of the section along the section-line 1F-1F, which is smaller than the drill diameter, i.e. the external diameter of the section along the section-line 1E-1E. Therefore, during the duration of drilling, the microdrill 50′ can effectively reduce the contact area between the drill body 54′ and the wall of a drilled hole, thereby reducing the heat generated during drilling, and enhancing the quality of the drilled hole. Such a microdrill 50′ is quite suitable to the drilling task for multi-layered PCBs.
In view of FIG. 1C, FIG. 1E and FIG. 1F, the sectional contour of the ST type microdrill 50 is different from that of the UC type microdrill 50′. The sectional contours of the two helical flutes 58 of the microdrill 50 are concave curves, so the web thickness 62 of the microdrill 50 is theoretically equal to the diameter of a minimum common tangent circle of the sectional contours of the two helical flutes 58 (i.e. the shortest distance between the sectional contours of the two helical flutes 58) and may be easily measured. On the other hand, since the drill body 54′ of the microdrill 50′ is ground to yield a UC diameter on the drill body 54′ and is fluted with the grinding wheel whose wheel contour is made up of combinations of multiple arcs, the sectional contours of the two helical flutes 58′ of the microdrill 50′ are sigmoid or convex curves. The web thickness of the microdrill 50′ is theoretically equal to the diameter of a maximum common tangent circle of the sectional contours of the two helical flutes 58′. Therefore, the web thickness of the microdrill 50′ has such a complicated definition and is not easily measured.
The web thickness measuring methods of microdrills can be based on a non-destructive measuring technology or a destructive measuring technology in general. In practice, the non-destructive web thickness measuring method, such as the disclosure in the Taiwan Patent Publication No. I254124, still has problems such as high cost and insufficient stability, which limits the development of the non-destructive measuring technology of web thickness. Therefore, most manufacturers prefer using manual and destructive measuring technology of web thickness. The Taiwan Patent Publication No. I413756, i.e. the U.S. Patent Publication No. 20120236139, discloses a destructive web thickness measuring system of microdrills and a method thereof, which employ two vision modules disposed in different orientations to capture a microdrill in different directions to generate images presenting the microdrill, and then sequentially perform a positioning procedure, a grinding procedure, and an image computing procedure on the images of the microdrill to calculate the web thickness of the microdrill at a sectional position to be measured of the microdrill. However, such a disclosure in the Taiwan Patent Publication No. I413756 is applied to only ST type microdrills but not to UC type microdrills, that is, this disclosure has limits on the types of microdrills. Moreover, the disclosure in the Taiwan Patent Publication No. I413756 has to use two vision modules, resulting in more cost.